Defrost Failure or Moisture Intrusion
Frost forms when humid air meets the cold evaporator and freezes. A working freezer runs a defrost cycle several times a day to melt that frost off the coil and drain it away. When frost accumulates instead, either the defrost cycle has stopped clearing the coil, or more humid air is getting in than the cycle can handle. The first is a defrost heater, thermostat, or control problem. The second is a door seal, a door left open, or a gasket that no longer closes tightly. We confirm which by checking the coil, the heater circuit, and the seal.
Most Common Causes
- Defrost heater that has burned out and no longer melts the coil
- Defrost thermostat or bimetal that will not close the heater circuit
- Control board or timer not initiating the defrost cycle
- Door gasket that has hardened and no longer seals
- Door left ajar, or an item blocking it from closing
- Blocked defrost drain refreezing melt into ice
- High household humidity overwhelming a marginal seal
Notes by Brand
On Sub-Zero built-ins, frost on the freezer coil is a classic defrost-heater or thermostat failure, and because the fresh-food side depends on that airflow, the fridge can warm at the same time. Viking and Thermador units frost more often from a hardened gasket in the tight column doors. Liebherr refrigerators may flag a temperature alarm as the coil ices. Monogram and KitchenAid models in the GE and Whirlpool families commonly need a defrost heater or thermostat. True Residential commercial-style units rely on strong door seals that stiffen with age.
Symptom-To-Cause Reference
| Pattern | Likely Cause | Next Step |
|---|---|---|
| Ice sheet on back wall | Defrost heater or thermostat | Test and replace defrost part |
| Frost plus warm fridge | Coil iced, blocking airflow | Diagnose defrost cycle |
| Frost near the door | Hardened or torn gasket | Replace door gasket |
| Ice in the drain area | Blocked defrost drain | Clear and heat the drain |
| Frost returns after thaw | Control or timer fault | Repair defrost control |
Do Not Chip Ice Off the Coil
Chipping frost with a knife or scraper can puncture the evaporator and cause a refrigerant leak, turning a moderate defrost repair into a sealed-system job. Thaw gently or book service to clear it properly.
Book a Frost Diagnosis
Diagnostic and travel fees, if applicable, are confirmed before the appointment and before repair approval.